This invention relates generally to a tool for use in the bonding of wire to semiconductor devices and, more particularly to a bonding tool having controlled attenuation characteristics.
Modern electronic equipment relies heavily on printed circuit boards on which semiconductor chips, or integrated circuits (ICs), are mounted. The mechanical and electrical connections between the chip and the substrate have posed challenges for chip designers. Three well known techniques for interconnecting the IC to the substrate are: wire bonding, tape automated bonding (TAB) and flip-chip.
The most common of these processes is wire bonding. In wire bonding, a plurality of bonding pads are located in a pattern on the top surface of the substrate, with the chip mounted in the center of the pattern of bonding pads, and the top surface of the chip facing away from the top surface of the substrate. Fine wires (which may be aluminum or gold wires) are connected between the contacts on the top surface of the chip and the contacts on the top surface of the substrate. Particularly, the connecting wires are supplied and bonded to the chip and to the substrate through a capillary, a bonding tool further described below.
Capillaries are used for ball bonding the wire to electronic devices, particularly to bond pads of semiconductor devices. Such capillaries are generally formed from a ceramic material, principally aluminum oxide, tungsten carbide, ruby, zircon toughened alumina (ZTA), alumina toughened zircon (ATZ) and other materials. Very thin wire, generally on the order of about one mil gold, copper or aluminum wire, is threaded through an axial passage in the capillary with a small ball being formed at the end of the wire, the ball being disposed external of the capillary tip. The initial object is to bond the ball to a pad on the semiconductor device and then to bond a portion farther along the wire to a lead frame or the like. During the bonding cycle, the capillaries perform more than one function.
After the ball is formed, the capillary must first center the ball partly within the capillary for bond pad targeting. With a first bonding step, the ball is bonded to a pad on a semiconductor device. When the capillary touches the ball down on the bond pad, the ball will be squashed and flatten out. As the bond pads are generally made from aluminum, a thin oxide forms on the surface of the bond pad. In order to form a proper bond, it is preferable to break the oxide surface and expose the aluminum surface. An effective way of breaking the oxide is to xe2x80x9cscrubxe2x80x9d the surface of the oxide with the wire ball. The wire ball is placed on the surface of the aluminum oxide and the capillary rapidly moves in a linear direction based on the expansion and contraction of a piezo-electric element placed within the ultrasonic horn to which the capillary is attached. The rapid motion, in addition to heat applied through the bond pad, forms an effective bond between the wire and the bond pad.
The capillary then handles the wire during looping, smoothly feeding the bond wire both out of the capillary and then back into the capillary. The capillary then forms a xe2x80x9cstitchxe2x80x9d bond and a xe2x80x9ctackxe2x80x9d or xe2x80x9ctailxe2x80x9d bond.
Presently, thermosonic wire bonding is the process of choice for the interconnection of semiconductor devices to their supporting substrates. The thermosonic bonding process is partially dependent upon the transfer of ultrasonic energy from the transducer, attached to a movable bondhead, through a tool, e.g. capillary or wedge, to the ball or wire being welded to the semiconducting device or supporting substrate.
In conventional capillaries (bonding tools), the geometry of the bonding tool is not engineered to modify energy transfer to the ball/wire interconnection pad interfacial area. The inventors of the present invention have determined that control of the ultrasonic attenuation of the tool is crucial to controlling the bonding process and its performance.
Conventional bonding tool design is deficient, however, because conventional bonding tool design is based on interconnection pitch and wire bond loop height and does not consider controlling ultrasonic attenuation.
FIG. 1 is an illustration of a conventional bonding tool. As shown in FIG. 1, bonding tool 100 has a cylindrical body portion 102 and a tapered portion 104. An axial passage 108 extends from the end 110 to the tip 106 of the bonding tool 100. A bonding wire (not shown) passes through axial passage 108 and through tip 106 for eventual bonding on a substrate (not shown).
To solve the aforementioned disadvantages of conventional bonding tools, the present invention relates to a bonding tool that produces controlled direction and gain of tool attenuation.
The bonding tool comprises a first cylindrical section having a substantially uniform first diameter; a second cylindrical section having a first end coupled to an end of the first cylindrical section, the second cylindrical section having a substantially uniform second diameter less than the first diameter; and a third section having a predetermined taper, a first end of the third section coupled to an end of the second cylindrical section.
According to another aspect of the present invention, the bonding tool comprises a first cylindrical section having a substantially uniform first diameter; a second section having a first end coupled to a first end of the first cylindrical section, the second cylindrical section having i) a diameter substantially equal to the first diameter of the first cylindrical section and ii) a planar area along at least a portion of a length of the second section; and a third section having a predetermined taper, a first end of the third section coupled to an end of the second cylindrical section.
According to yet another aspect of the present invention, the bonding tool comprises a first section having a substantially uniform first diameter, the first section having a planar portion formed along at least a portion of a length of the first section; a second cylindrical section having a first end coupled to an end of the first section, the second cylindrical section having a substantially uniform second diameter about equal to the first diameter; and a third section having a predetermined taper, a first end of the third section coupled to a second end of the second cylindrical section.
According to a further aspect of the present invention, the bonding tool comprises a first cylindrical section having a substantially uniform first diameter; and a second cylindrical section having a first end coupled to an end of the first cylindrical section, the second cylindrical section having i) a substantially uniform second diameter less than the first diameter and ii) a planar area along at least a portion of a length of the second section.
According to one aspect of the present invention, the bonding tool is formed from a unitary piece of material.
According to another aspect of the present invention, a transition section is coupled between the first section and the second section.
According to a further aspect of the present invention, the tapered section has a further tapered section at an end thereof.
According to yet another aspect of the present invention, a positioning guide is disposed at a second end of the first section of the bonding tool.
These and other aspects of the invention are set forth below with reference to the drawings and the description of exemplary embodiments of the invention.